• EC • Season Cracking • Caustic Embrittlement • SCC • SSC • LME • HB-HE-HIC • Fatigue • Erosion • Fretting • Stray Current • Index
 

Different Types of Corrosion
- Recognition, Mechanisms & Prevention

Stress Corrosion Cracking (SCC)
 

Recognition
   

What is stress corrosion cracking? Stress-corrosion cracking (SCC) is a cracking process that requires the simultaneous action of a corrodent and sustained tensile stress. This excludes corrosion-reduced sections that fail by fast fracture. It also excludes intercrystalline or transcrystalline corrosion, which can disintegrate an alloy without applied or residual stress. Stress-corrosion cracking may occur in combination with hydrogen embrittlement.

The image of stress corrosion I see
Is that of a huge unwanted tree
Against whose trunk we chop and chop,
But which outgrows the chips that drop;
And from each gash made in its bark
A new branch grows to make more dark
The shade of ignorance around its base,
Where scientists toil with puzzled face.
(by S P Rideout 1967)
  Mechanisms
   

What causes stress corrosion cracking? Stress corrosion cracking results from the conjoint action of three components: (1) a susceptible material; (2) a specific chemical species (environment) and (3) tensile stress. For example, copper and its alloys are susceptible to ammonia compounds, mild steels are susceptible to alkalis and stainless steels are susceptible to chlorides.

There is no unified mechanism for stress corrosion cracking in the literature. Various models have been proposed which include the following:

  • Adsorption model: specific chemical species adsorbs on the crack surface and lowers the fracture stress.

  • Film rupture model: stress ruptures the passive film locally and sets up an active-passive cell. Newly formed passive film is ruptured again under stress and the cycle continues until failure.

  • Pre-existing active path model: Pre-existing path such as grain boundaries where intermetallics and compounds are formed.

  • Embrittlement model: Hydrogen embrittlement is a major mechanism of SCC for steels and other alloys such as titanium. Hydrogen atoms diffuse to the crack tip and embrittle the metal.
  Prevention
    How to prevent stress corrosion cracking? Stress corrosion cracking can be prevented through:
  • Control of stress level (residual or load) and hardness.
  • Avoid the chemical species that causes SCC.
  • Use of materials known not to crack in the specified environment.
  • Control temperature and or potential
  For more details
    More details on stress corrosion cracking are included in the following corrosion short courses which you can take as in-house training courses, online courses or distance learning courses:

Corrosion and Its Prevention (5-day module)
 Corrosion and Its Prevention (2-day module)
 Corrosion, Metallurgy, Failure Analysis and Prevention (3 days)
 Marine Corrosion, Causes and Prevention (2 days)
 Materials Selection and Corrosion (2 days)
 Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail (2 days)
     
Home | Subject Index | Contact Us

Copyright © 1995-2009. All rights reserved.